Regulatory T-cells (Treg cells; also known as suppressor T-cells) are subpopulations of T-cells that maintain immune homeostasis and help avert autoimmune disease (Sakaguchi et al., 2008; Shevach, 2006; Vignali et al., 2008). Interest in Treg cells is focused predominantly on prototypic CD4+ CD25+ Treg cells that are programmed by the transcription factor FoxP3 (Fontenot et al., 2003; Hori et al., 2003). In resting polyspecific populations these Treg cells are characterised in the mouse both as ‘natural’, thymus-derived and induced ‘adaptive’ cells that suppress the activation, proliferation and functions of other T-cells (Sakaguchi et al., 2008; Shevach, 2006). However, in human blood CD4+ Treg cells are not as reliably distinguished by FoxP3 expression (Roncarolo and Gregori, 2008; Allan et al., 2007; Gavin et al., 2006). Thus, CD4+ T-cells with markers of either naïve or memory cells were shown to have similar suppressor functions despite low and high expression, respectively, of FoxP3 (Miyara et al., 2009). Other surface markers of human CD4+ CD25+ FoxP3+ Treg cells such as decreased expression of the IL-7 receptor, CD127 (Liu et al., 2006; Seddiki et al., 2006), are not specific for Treg cells.
Aside from the paucity of specific cell surface markers, the mechanisms underlying suppression by CD4+ CD25+ FoxP3+ Treg cells remain controversial. In the mouse, suppression ex vivo has been shown to require cell-cell contact but has been attributed to multiple mechanisms (Vignali et al., 2008; Shevach, 2009; Sakaguchi et al., 2009); even less is known about the function of similar human Treg cells. Furthermore, other types of both CD4+ and CD8+ Treg cells that differ in proposed mechanisms of suppressor function have been described in the context of various tissue sites or diseases (Vignali et al., 2008).
Treg cells induced by administration of autoantigens have been shown to protect against some autoimmune diseases in certain animal models (reviewed by von Herrath and Harrison, 2003). For example, in the nonobese diabetic (NOD) mouse model of type 1 diabetes (T1D) CD4+ Treg cells induced by administered pancreatic islet autoantigens such as insulin (Bergerot et al., 1994) or glutamic acid decarboxylase 65 (GAD65) (Tisch et al., 1999), or transfer of CD4+ Treg cells induced by proinsulin (Every et al., 2006) or a putative pancreatic islet antigen (Tang et al., 2004), have been shown to protect against autoimmune diabetes. However, in these models Treg cells have been studied in resting, polyspecific populations and not during the host's response to a particular antigen. Recently, proinsulin- and GAD65-specific human CD4+ T-cell clones were generated and Treg clones were distinguished by their suppressor function in vitro (Dromey et al., 2011). The cell surface membrane-anchored glycoprotein CD52 was shown to be upregulated in these expanded CD4+ Treg clones. However, the mechanism of immune suppression has not previously been characterized.